1. Field of the Invention
The field of the invention generally relates to a method and apparatus for cutting silicon ingots to produce silicon wafers. In particular, the field of the invention relates to an improved wire saw comprising the placement of a stabilizing strip for holding adjacent wafers in the sawing process to stabilize the wafers against vibratory effects and facilitate automated processing of the finished wafers. The stabilizing strip enables cutting of ultra thin silicon wafers with a conventional process, resulting in low kerf loss, improved material utilization, minimized total thickness variation, and thus greater cost effectiveness.
2. Background of Related Art
Conventional wire saws or wire-webs for slicing silicon are well known. Such wire saws typically comprise a row of fine, high tensile strength wires having diameters on the order of 0.1–0.2 millimeters. The wires are disposed in parallel with one another and are translated in the same direction. A workpiece is pressed against these wires. At the same time, an abrasive suspension fluid is supplied between the work piece and the wires, enabling the wires to slice the workpiece into wafers by an abrasive grinding action. The liquid suspended abrasive particles are provided onto the moving “web” or wire through a circulation system that places a blanket like coating of the abrasive suspensions onto the “web” just before the wire-web impacts the work piece. The abrasive particles carried by the liquid are transferred via the coated wires to produce a grinding or cutting effect.
More recently, diamond coated wires are employed in wire saws in an attempt to increase the rate of cutting of silicon wafers. The workpiece is pressed against the diamond wire and the cutting process is augmented by diamond particles embedded in the wire. However, due to their smaller core diameter, diamond saw wires are more fragile. Such mechanical sensitivity promotes damage and cracks in the wires at tensioning and guide rollers.
In a conventional wire sawing process, the wire is high tensile strength brass plated steel wire, and the actual cutting is done in a slurry consisting of oil or polyethylene glycol and silicon carbide. Since this is a free abrasive process, undesirably high wire speeds are required. Also, large quantities of slurry are required for slicing and cooling. Because of this, strong hydraulic forces are applied to the wafers being cut creating problems when slicing thin wafers. Since a great amount of process stress is applied to the wafers, there is a further problem in that residual process distortion becomes great.
U.S. Pat. No. 5,937,844 describes how a conventional wire saw process using a slurry results in a variation of the rate of transport of abrasive grains as the wire web cuts down through the ingot. Accordingly, there is a need to adjust the rate of feed of slurry or vary viscosity.
U.S. Pat. No. 5,099,820 discloses an abrasive liquid as a suspension of particles of silicon carbide in water or oil. However, such conventional suspensions are not stable and do not provide uniform coating on the cutting wires. Furthermore, such compositions require vigorous agitation to maintain uniform suspension of the particles, and the suspension settles out quickly under stagnant conditions, and even during workpiece slicing while still under agitation.
Achieving an optimum cutting quality depends on a combination of parameters, the quality (lubricity, viscosity, tack properties, etc.) of the abrasive fluid and the force with which the workpiece is pressed against the set of free abrasive or diamond coated wires. In a conventional wire saw, silicon wafers consisting of brittle material are cut with wire characterized by high tensile strength and hardness. When cutting is done using conventional adhering free abrasive particles, or diamond wire, an extreme amount of process stress is applied to the wafers. The force of the wires against the workpiece can deform the workpiece and degrade planarity characteristics in the resulting wafer, thus adding to the need for further processing time and adding to overall cost.
It also has been found that a conventional free abrasive wire or diamond-coated wire sawing process tends to cause the wafers to oscillate and to deform during the sawing process. When slicing very thin wafers, one of the problems encountered is that as the wires progress downward through the ingot, unsupported sections of the wafers tend to vibrate, move, or stick together. This disadvantageously imposes a limit on the thickness of wafers that can be achieved through a current mass production wire saw process. Vibration and oscillation of the wafers also contribute to surface damage of the wafer such as wire marks that are difficult to remove and adversely affect wafer performance. Vibration and oscillation of the wafers also contribute to stress applied to the wire and limit diameter reduction of the steel strength member, limiting kerf reduction and providing sub-optimal material utilization.
Mass production considerations, such as the rate of wear of the wire, the effectiveness, recovery and recycling of the cutting and lubricating fluids, are important factors in achieving high cutting quality at a reasonable cost. Cutting quality typically refers to the ability to provide exact planarity of surfaces without taper, bow, warp, thickness variation and surface damage to yield products suitable as a starting base for advanced semiconductor devices and solar cells.
For many applications, ultra thin wafers of substantially uniform thickness, low warp and low bow are desired. For high efficiency, long life solar cells, precise planar dimensions are critical in the formation of a starting wafer to provide a predictable, stable base for the subsequent processes such as diffusion, anti-reflective coatings and thermal processes. Conventional wire saws have disadvantages in attempting to provide a cost effective way to cut silicon into very thin wafers, with thicknesses down to 200 microns or less, that would be suitable for use in solar cells. Imperfections due to process distortion or defects in planarity, warp, bow, variations in thickness and surface damage are still too prevalent to achieve cost effective mass production of ultra thin silicon wafers suitable as a starting base for a high efficiency low cost solar cell.
Therefore, what is needed is a wire saw and cutting system that can optimize the cutting quality that can be obtained on silicon under mass production conditions. What is also needed is a wire saw system that can apply an optimum cutting pressure to the wafers and eliminate process distortion. Such a system advantageously would enable cutting of thinner, lightweight wafers with improved control and stabilization. Such a system ideally would minimize total thickness variation (TTV), provide substantially uniform planarity, and substantially eliminate bow and warp. Optimum cutting pressure to the wafers also reduces stress on the wire and enables use of thinner wires that reduces kerf losses and increases material utilization, contributing to lower cost. Such ultra thin, uniform silicon wafer, that can be mass-produced at reasonable cost would be especially useful as a starting material for a high efficiency solar cell.
There exists a need for a novel cutting and lubricating composition that would provide a uniform supply of homogeneously dispersed abrasive particles, attached to and traveling with the wire, without abrasive particle agglomeration or “hard-cake” formation from suspension fallout. Such a cutting/lubricating composition advantageously would enable a workpiece to be cut more efficiently, requiring less cutting pressure and minimizing distortion. Further, the cutting composition should have excellent lubricity and heat transfer properties to remove the frictional heat generated at the cutting site thereby increasing working life of the wire and avoiding process downtime. It is also desirable that the composition should provide a stable suspension of abrasive particles.
When the ultra thin wafers (having a thickness down to the order of 200 microns or less) are released from the wire saw, it is imperative to keep the wafers from adhering to one another to avoid damage when placing the wafers into cassettes for further processing operations. Thus, there is also a need for automated handling of the released wafers in a non-contacting arrangement for subsequent transport and insertion into cassettes for final processing. It especially would be desirable to provide a means for automated positioning of wafers into cassettes of varying dimensions to facilitate further processing operations.